This invention relates to a method and apparatus for separating air.
The most important method commercially of separating air is by rectification. The most frequently used air separation cycles include the steps of compressing a stream of air, purifying the resulting stream of compressed air by removing water vapour and carbon dioxide, and pre-cooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column i.e. one of the two columns operates at higher pressure than the other. Most if not all of the air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and liquid nitrogen vapour. The nitrogen vapour is condensed. A part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column, is sub-cooled, and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid is separated into substantially pure oxygen and nitrogen products in the lower pressure column. These products are withdrawn in the vapour state from the lower pressure column and form the returning streams against which the incoming air stream is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure cola, sub-cooling it, and passing it into the top of the lower pressure column through a throttling or pressure reduction valve.
Conventionally, the lower pressure column is operated at pressures in the range of 1 to 1.5 bar. (Unless stated to the contrary, all pressures given herein are absolute, and not gauge, pressures.) Liquid oxygen at the bottom of the lower pressure column is used to meet the condensation duty at the top of the higher pressure column. Accordingly, nitrogen vapour from the top of higher pressure column is heat exchanged with liquid oxygen in the bottom of the lower pressure column. Sufficient liquid oxygen is able to be evaporated thereby to meet the requirements of the lower pressure column for reboil and to enable a good yield of gaseous oxygen product to be achieved. The pressure at the top of the higher pressure column and hence the pressure to which the incoming air is compressed are arranged to be such that the temperature of the condensing nitrogen is a degree or two Kelvin higher than that of the boiling oxygen in the lower pressure column. In consequence of these relationships, it is not generally possible to operate the higher pressure column below a pressure of about 5 bar.
It is also possible to operate the lower pressure column at more elevated pressures. If the operating pressure of the lower pressure rectifier is so raised, there is a consequential increase in the pressure at which the higher pressure column is operated.
Improvements to the air separation process enabling pressure ratio between the higher pressure column and the lower pressure column have been proposed in order to produce an impure oxygen product, containing, say, from 3 to 20% by volume of impurities. U.S. Pat. No. 4,410,343 discloses that when such lower purity oxygen is required, rather than having the above-described link between the lower and higher pressure columns, air is employed to boil oxygen in the bottom of the lower pressure column in order both to provide reboil for that column and to evaporate the oxygen product. The resulting condensed air is then fed into both the higher pressure and the lower pressure columns. A stream of oxygen-enriched liquid is withdrawn from the higher pressure column, is passed through a throttling valve and a part of it is used to perform the nitrogen condensing duty at the top of the higher pressure column.
U.S. Pat. No. 3,210,951 also discloses a process for producing impure oxygen in which air is employed to boil oxygen in the bottom of the lower pressure column in order both to provide reboil for that column and to evaporate the oxygen product. In this instance, however, oxygen-enriched liquid from an intermediate region of the lower pressure column is used to fulfil the duty of condensing nitrogen vapour produced in the higher pressure column. This process is capable of reducing the operating pressure of the higher pressure column close to 4 bar.
The methods disclosed in U.S. Pat. No. 3,210,951 and U.S. Pat. No. 410,343 become less suitable for use if the lower pressure column is to be operated at a pressure in excess of about 1.5 bar.
EP-A-0,538,118 discloses a method of operating a double column process above the conventional pressure limits without loss of oxygen recovery and with improvements in power consumption. In one example, oxygen-enriched liquid air is taken from the bottom of the higher pressure rectification column and is introduced into a further column at a level above all the liquid-vapour mass exchange surfaces therein. The further column operates at pressures intermediate those in the higher pressure column and those in the lower pressure column. The further column provides a liquid feed and a vapour feed to intermediate levels of the lower pressure rectification column.
Our European patent application 94302953.8 to be published on 11 Jan. 1995 under the number EP-A-0,633,438 discloses with reference to its FIG. 2 a process broadly similar to that shown in the drawing accompanying this application save that the impure oxygen product is vaporised by heat exchange with nitrogen withdrawn from the higher pressure rectification column. A disadvantage of this arrangement is that if the process is operated at a pressure in the lower pressure rectifier much above 5 bar the product recovery (i.e. the yield of oxygen) falls. There is an increasing demand for high pressure nitrogen product in so-called integrated gasification-combined cycle (IGCC) processes, the nitrogen being supplied to the combustion chamber or expander of a gas turbine which generates power by combustion of a fuel gas which is a product of the gasification. The oxygen product of the air separation is itself used as a reactant in the generation of the fuel gas. It is therefore advantageous to operate the lower pressure rectifier at pressures in the range of, say, 5 to 10 bar without there being a reduction in the yield of oxygen.
The present invention aims at providing a method and apparatus which are able to achieve this advantage.